2011-08-29_REVISION - M1986002 ‎(13)‎

• Permit No: M1986002
• IBM Index Class Name: REVISION
• Doc Date: 8/29/2011
• Doc Name: Amendment to CN-01
• From: Greg Lewicki and Associates
• To: DRMS
• Type & Sequence: CN1
• Email Name: THM
• Media Type: D
• Archive: No

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Craig Ranch Pit — General Stormwater Comments
Page 3
June 23, 2011
ii. If the sandstone is not rippable, consider keeping the proposed islands out
of the existing drainages such that the vegetative medium is out of the
100 -year flow path and less susceptible to erosion.
iii. The upgradient contact edge of the islands with the sandstone should be
parallel to elevation contours such that runoff tends to pond against the
island rather than flowing along the contact edge and eroding the island
material.
iv. Consider constructing collection/divcrsion ditches along the upgradient
edge of the islands.
4. Page E-4, Post - Reclamation Site Drainage The text states "All onsite flows will end up
in a sediment pond either in Mining Area 1, 2 or 4." Map F does not show a sediment
pond in Mining Area 2. The Applicant shall provide:
a. A location of the sediment pond in Mining Area 2 on Map F and on Map C -2 (if
applicable).
b. Sediment pond sizing for the sediment pond in Mining Area 2 as discussed in
Comment No. 2.
o. Sufficient information (e.g., hydrologic analyses and hydraulic computations) for
the Division to evaluate the collector ditch sizing used to divert runoff to the
Mining Area 2 sediment pond. Please note because these ditches are to be part of
the post mine land use, the Division recommends they be sized to convey runoff
from the 100 -year, 24 -hour storm event, not the PMP. Also note Comment No. 5
below.
5. Page E-5, first paragraph/precipitation — The Applicant states the 100 -year, 24 -hour
precipitation event is 2.2 inches. No reference is provided, but it appears the Applicant is
using Figure S -7 in "Procedures for Determining Peak Flows in Colorado ". The
isopluvial reaps in that document are outdated. Current 24 -hour precipitation depths
should be obtained from NOAA Atlas II, Volume 3, available online at:
littp://lidsc.nws.noaa.gov/hdsc/pfds/other/co_pfds,litml, and specifically at
b ttp : / /www.nws.noaa,gov /oh/hdsc/PF documents/Atlas2 Volume3.pol
The Division estimates the 100 -year, 24 -hour stone depth to be 2.6 inches for the area.
Please correct all calculations accordingly.
6. Page 13 -5, first paragraph/Curve Number — The Applicant states a conservative curve
ntunber (CN) of 85 was used for calculations. Based on the areas provided in Table E -1
(page E -1), the Division estimates a composite curve ntunber between 87 and 88,
assuming 82.88 acres (or 41 percent) of the 200 contributing acres are impervious
sandstone. The Applicant shall use CN = 88 to be conservative, or provide rationale for
the CN = 85.
7. Page G -1, Hydrology and Sediment Control — The Post - Reclamation Site Drainage
section on page E-4 combined with Map F indicates there are at least four sediment
ponds (one large pond in Mining Area 6, two small ponds in Mining Area 1, and one
unknown pond in Mining Area 2). The calculation on page 0-1 is for one pond.
m:VNnto1 m.t988.002 cralg rancMs omiwatercommonts _mem21tun11.docx